The present invention relates to apparatus and methods for continuously monitoring the operability of individual channels in a carrier telephone system.
Carrier telephone systems are characterized by the use of different carrier frequencies for each subscriber so that multiple subscribers can share a common transmission line between the subscriber stations and the central office. Such systems typically employ a central office terminal for each subscriber circuit. Information transmitted from each central office terminal to its associated subscriber circuit employs a unique carrier frequency; likewise, transmission from each subscriber circuit to its associated central office terminal utilizes another unique carrier frequency. Thus if twelve subscribers share a transmission line, there would be twelve unique central office carrier frequencies (for transmission to the subscribers) and twelve unique subscriber carrier frequencies (for transmission to the central office).
The industry has long sought a continuous automatic monitoring arrangement for carrier systems whereby failure or inoperability of any subscriber channel would be automatically indicated. Numerous manually-initiated operability testing arrangements have been suggested, typified by that disclosed in U.S. Pat. No. 3,840,706 to Krasin et al. However, a continuous automatic approach, if feasible, would have the advantages of freeing personnel for other duties and eliminating the periods of unrecognized inoperability which may exist between manually-initiated tests. An automatic continuous monitoring approach also permits the telephone company to become aware of an inoperable channel before the subscriber, thereby permitting the channel to be repaired and put back in service without the subscriber ever knowing the channel was not operating.
The major problem in providing an automatic continuous monitoring system for carrier telephone systems derives from the subscriber on-hook conditions. During off-hook condition the subscriber carrier is transmitted in most carrier systems, and this carrier can be monitored at the central office terminal. However, for on-hook conditions, the subscriber carrier is generally not transmitted. Transmission of test signals back to the central office is therefore not possible during on-hook conditions without considerable modifications to existing equipment. Such modification is both time consuming and expensive, particularly where the subscriber circuit must be extensively modified.
One approach to automtically continuously monitoring carrier system operation is suggested in U.S. Pat. No. 2,572,030 to Jacobsen et al. In that patent, the testing arrangement is limited to carrier systems of the suppressed carrier type (i.e. -- where one or both sidebands but not the carrier is transmitted between the central office terminal and the subscriber circuit). The patent recognizes that there is always some carrier leakage in suppressed carrier systems and that this leaked carrier signal can be used to monitor channel operability. If the leaked carrier is not received, or is received at an abnormally low level, a failure indicator is actuated. The limitation of this approach to suppressed carrier systems makes it unavailable for most carrier telephone systems in operation today. Moreover, this approach only monitors carrier frequency circuits in the channel; it does not monitor the voice frequency circuits which, if inoperable, present just as much of a problem as the carrier frequency circuits.
It is therefore an object of the present invention to provide a method and apparatus for automatically and continuously monitoring the operability of a carrier telephone system.
It is another object of the present invention to provide a method and apparatus for automatically and continuously monitoring the operability of both carrier and voice frequency circuits in a carrier telephone system.